Pre or post process Gamma correction?

[valnar]

I'm curious as to how others on this forum handle gamma correction on their DivX's. I do not output back to TV, so I watch all my DivX movies on my PC monitor. Because of the gamma difference between NTSC and most PC video cards/monitors, a boost of 1.2 or 1.3 in gamma is often required otherwise the DivX will play too dark.

For those that watch DivX movies on their PC ----

Do you adjust the gamma on playback with your video drivers, or do you bake it in the DivX during compression/editing?

Robert

[DJ Bobo]

Never done such a thing ¬.¬

[jonny]

I agree, never

[jggimi]

Valnar, I hope you don't mind, I added a "Never" to your poll, since both DJ and Jonny mentioned it, and there might be many more "Never" people out there.

[valnar]

No problem on the poll. I tried to change it myself... :(
I ask because I'm looking at all my PC's in the house, and less than half have overlay adjustments with gamma. My ATI Radeon cards are fine, but I have a very new GeForce 4400Ti that has no such adjustment! I can only control the standard brightness/contast/hue, but changing those values wash out colors. My DivX encodes where I adjusted the gamma beforehand look great on my GeForce PC and older video cards- with no such post processing needed.

My worry is how portable my DivX files would be should I need to view them on a MacIntosh or another computer with a different default gamma level.

I'd ask what the standard was in the Industry, except we are no industry here. Just us Doom9 forum people. :)

Hence, the consensus.

Robert

[cordraconis]

A friend of mine who lives just a few rooms further in my student dorm, mentioned a thread on one of the forums he regularly posts in (I think it was on FlipCode or something ... I'll ask him tomorrow, I'ts rather late now.)
His name is Nicolas "Nick" Capens. BTW, maybe some of the programmers around here knows him or at least which forums he regularly visits ... He's also the programmer of SoftWire on Sourceforge.

The thread was about something with Anti-Aliasing, and the votes (for the screenshots) were totally off. So then they started to discuss this "Gamma correction"-thing, and one of them posted a link to a file for tuning the colors of your monitor. The link was here;
(dont forget to enlarge the picture, since IE automatically fits it to the window.)

http://simonnihal.homestead.com/files/assorted3d/gamma-2-2.png

(Don't know all the details about checking this link by a moderator :D)

I used this to adjust my color channels. (Using the nForce drivers from Nvidia and *not* from microsoft, since these would not allow me to change the channels individually). (And also the screen temperature on my Iiyama HM704 was set to 9000K or something, and so I changed it to 6500K (Which it should be).)

From what I understand about this, the gamma correction is solely about the monitor, so I don't really understand why one would hard-code it into his divX-es. That's why I voted post-process.

P.S. The colors of Nicolas' and my screen are now identical, so for comparing screenshots, It's perfect. I suggest everybody should do this.

[dTb]

I've also never felt the need to adjust gamma, maybe if I was watching something and the light in the room was too bright I would adjust it in the decoding filter.

@cordraconis, in what way are you supposed to adjust things using that image?

[valnar]

Huh. I'm surprised at all the "never" responses. Its a known fact that the gamma/brightness of a monitor is less than a TV. Do you just watch it darker than the original?

Robert

[cordraconis]

Oh, I see ... :D

Right-click on your desktop, and with "settings > Advanced" go to the
"Color correction" of your video card.

Then open and expand the image, and select a color (for example Red)in your videocard settings. Adjust the "red" slider untill the square and the "darker box around that square" have equal brightness for that color. Continue with the other colors.
If you have done it correctly, then the white squares should also have equal brightness in respect to ther "boxes".

(You might have to take some distance from your screen and squeeze your eyes to see it better ;) )

Happy tweaking ... !

[temporance]

Originally posted by valnar
Huh. I'm surprised at all the "never" responses. Its a known fact that the gamma/brightness of a monitor is less than a TV.There is actually no difference between the gamma of monitors and TVs. Display device gamma is a function of the phosphors used in the tubes and is around 2.5 for both types of device.

The problem is that computer graphics cards apply a gamma correction so that they can have linear intensity images in their video memory. This is normally helpful for things like 3d games but is not good for video. Video signals are already gamma corrected in the camera so need to be treated differently when shown on a PC.

If you want the information from the horse's mouth, see: http://people.ee.ethz.ch/~buc/brechbuehler/mirror/color/GammaFAQ.html#RTFToC13

BTW, the only "right" place to adjust gamma in typical DivX ripping is at the decoder (or better still in the graphics card at playback).
Edit: what would really help is some of those gamma testcards in DivX format so that we could all see how good our displays are!!

[cordraconis]

Originally posted by temporance

Edit: what would really help is some of those gamma testcards in DivX format so that we could all see how good our displays are!!

:D
Hit me ... I'm prepared. ;) (Then I can see if all that changing of color levels was worth the trouble.